Imaging of the CNS

Question 1

Attenuation in X-rays

Answer 1

The greater the tissue density, the more the X-rays are attenuated, so fewer reach the film. Dense tissue appears white.

Question 2

5 radiographic densities

Answer 2

Air - black
Fat - dark grey
Muscle - grey
Bone - light grey
Metal - white

Question 3

Role of X-rays for imaging the bone in the CNS

Answer 3

Fractures
Arthritis
Metastasis
Osteoporosis

Question 4

Name 3 advantages/disadvantages of X-rays

Answer 4

Fast
Cheap
Good bone detail
Dynamic images (serial x-rays)

Poor soft tissue detail
X-ray dose can cause harm
Images 2D

Question 5

Technique behind CT scans

Answer 5

X-ray tube and detectors circle around the body repeatedly and a stack of axial images of varying thickness is produced. Digital map of density is converted to greyscale

Question 6

Radiographic densities in CT

Answer 6

Air - black
Fat - black
Water - dark grey
Brain - grey
Blood - white
Bone - very white

Question 7

CT windows

Answer 7

Gradient of the greyscale is manipulated to emphasise differences between the different tissues.

Question 8

Name 2 advantages/disadvantages of CT scans

Answer 8

Excellent bone detail
Good soft tissue detail
Quiet
Can be used for biopsy
Gives 2D and 3D images

Expensive
High x-ray dose

Question 9

Role of CT

Answer 9

Trauma - haematomas, fractures
Stroke - haemorrhage
Severe headache - space occupying lesion
Unconscious patients
CT guided biopsy

Question 10

Technique of MRI

Answer 10

Protons align in a strong magnetic field. Radiofrequency pulses energizes protons. When the pulse is turned off, the protons relax and a radiofrequency signal is emitted. Signal intensity is used to create a 3D map.

Question 11

Types of MRI image

Answer 11

T1: fluid is dark
T2: fluid is light
FLAIR: T2 scan where the fluid has been attenuated (removes brightness of T2 scan)

Question 12

Name 3 advantages/disadvantages of MRI

Answer 12

No x-rays
Detailed anatomy
Good soft tissue detail
Multiple plane views

Very slow
Expensive
Claustrophobic and noisy
Poor bone detail

Question 13

Contraindications for MRI

Answer 13

Metal implants e.g. aneurysm clips, metal in eye
Foreign bodies
Pacemakers
Obese patients

Question 14

Use of MRI for CNS

Answer 14

Stroke - diffusion weighted
Structural lesions/malformations
Soft tissue
demyelinating disease
Looking at brainstem and cerebellum

Question 15

Uses of PET in CNS imaging

Answer 15

Dementia
Stroke
epilepsy

Question 16

Types of rhythm seen in an EEG

Answer 16

Alpha: 8-13Hz, eyes shut, meditation
Beta: >14Hz, parietal and frontal activity and tension, sleep spindles
Delta: <3.5Hz, deep sleep, coma

Gamma: 40Hz, learning and memory
Theta: 4-7Hz, parietal and temporal alertness, learning and memory.

Question 17

Causes of ring enhancing lesions in an MRI

Answer 17

Metastasis
Abscess
Glioma
Infract
Contusion

Lymphoma